The subject matter disclosed herein relates to on-load tap changers for high voltage devices, and specifically to on-load tap changers for a high power transformer utilizing micro-electromechanical system (MEMS) technology.
Currently, a complex mechanical switching assembly accomplishes on-load tap changers (OLTC). Mechanical OLTC mechanisms include an electric motor for charging powerful springs to open and close switches in the switching assembly of these OLTC mechanisms. The switches in the switching assembly are mechanically actuated on and off in a sequence coordinated by mechanical interlocks to orchestrate the switch openings and closings with the correct timing. These mechanical interlocks can bind and prevent switching from occurring. Although much development has been done to reduce switch contact electrical stress (such as reducing arcing when each switch opens), a main failure mode is switch contact failure. Furthermore, because the OLTC switch assembly has many integrated and mechanical moving parts, it has frequent problems and must be maintained regularly which can be costly. Furthermore, because the conventional OLTC switch assembly is immersed in an insulating media such as oil or SF6 gas to reduce the arcing problem, the maintenance on OLTC switch assembly can be costly and time consuming. Mechanical OLTC mechanisms are also large, slow and noisy, which may be undesirable. The mechanical moving parts of the conventional OLTC are the source of a significant portion of the problems in power transformers that include an OLTC.
Solid-state switching devices have been used to reduce a few failure modes, but are known to have other failures or disadvantages when used as a switching component in a transformer on-load tap changer application. It is well known that semiconductor switching means exhibit parasitic energy losses and undesirable off-state leaks. Semiconductor switches also have forward voltage drop even when they are on. When a semiconductor switch is in an open position it still lets through a little bit of current, which is undesirable. Although solid-state switches can provide high switching speeds, they suffer from significant power losses and can be very costly.
Accordingly, it is desirable to have an on-load tap changer for a high powered transformer using switching technology that is cost-effective and is capable of switching less than one micro-second and in a fashion to be arcless by diverting the energy. It is further desirable to have an on-load tap changer for a high-powered transformer using switching technology that can reduce or eliminate the switching failure modes of a conventional switch and eliminate the parasitic energy losses of a semiconducting switching means.